Cyclic meso-ionic compounds. Part 23. Novel chemistry of 1,2,4-thiadiazoles and their transformation into meso-ionic 1,2,4-thiadiazolium derivatives

Representatives of two new classes of meso-ionic heterocycles have been synthesised, the 1,2,4-thiadi-azolium-3-olate (8) and the 1,2,4-thiadiazolium-3-tosylaminide(25). The reactions of 1,2,4-thiadiazoles and nucleophiles follow two general pathways: (i) reductive transformation to N-thiobenzoyl derivatives and (ii) elimination of elemental sulphur and the formation of N-benzoyl derivatives. A mechanistic rationale is proposed for the operation of pathways (i) and (ii). Earlier views on the oxidative formation of certain 1,2,4-thiadiazoles are corrected. A novel synthetic route to heterocycles containing sulphur–nitrogen bonds is described. 1,2,4-Thiadiazoles are formed by oxidation of N-thiobenzoylureas and N-thiobenzoylguanidines by bis(4-methoxyphenyl) telluroxide.